1. Field of the Invention
The present invention pertains to a receiver system for locating transmitters and has particular utility for locating transmitters carried by miners trapped underground.
2. Discussion of the Prior Art
Underground mining is a hazardous occupation due to the continuous threat of being trapped underground by a cave-in. When such cave-ins occur, it is important to locate miners trapped underground in order to facilitate their rescue. Accordingly, miners have carried portable transmitters which can be energized in an emergency to produce a signal permitting people above the ground to determine the miners' locations.
One prior art method of locating an underground transmitter, such as a portable transmitter carried by a trapped miner, is to cause there to be a large pulse of current in a loop of wire laid out upon the ground and to detect the resulting magnetic field at the surface. The relative behavior of the horizontal and vertical components of this field makes it possible to locate a point on the surface above the location of the underground source. It is necessary to detect these signals in the presence of both natural and man-made noise, and the power which can be generated underground is limited by both safety considerations and by the available power under emergency conditions. In addition, detecting the location of the signals involves determining conditions of maxima, equality between the vertical and horizontal field components, and nulls in the horizontal field. The performance of this method is limited by the presence of noise and low power and must work under unfavorable signal-to-noise ratio (S/N) conditions. Furthermore, under actual emergency conditions there will frequently be several underground transmitters in operation, and it must be possible to examine them one at a time while the need for low cost in the underground units may cause there to be a relatively large tolerance in frequencies generated thereby.
A trade-off of the likely natural and man-made noise spectrum and of other factors, such as losses in passing through the ground, has led to the conclusion that the current pulses generated underground in the loop should occur at a relatively high rate, such as about 500 per second. These pulse signals could be sent continuously, but will more likely be sent in bursts, such as 1 second on and 5 seconds off, to conserve battery power underground. If these pulses are sent at a rate of f.sub.1 per second where f.sub.1 is in the order of 500/second, then the received signals at the surface will contain energy at f.sub.1, 2f.sub.1, 3f.sub.1, etc.
As a result of the poor S/N environment, the surface receiver must respond to the energy in several of these spectral lines, but must reject noise. There are several ways of achieving this requirement including delay-line and integrator techniques; however, the most simple method is to synthesize a comb filter from narrow band filters at frequencies f.sub.1 ; 2f.sub.1 ; 3f.sub.1 ; etc. These filters may be either at the above frequencies or at some other frequency sidestepped from them to serve the usual purposes of avoiding the need for an unmanageable amount of gain at one frequency and allowing the filters to be constructed at frequencies where this is most convenient. The bandwidth of each of these filters should be as narrow as possible but must be wide enough to tolerate frequency errors, both long and short term, in the underground transmitters and caused by the effects of transmission through the earth. The smallest bandwidth likely to be tolerable for the filters in about 10 Hz for the first filter and 20, 30, etc., Hz for succeeding filters. This simple receiver has two problems. The first problem is that if there are n underground sources operating at pulse rates of f.sub.1, f.sub.2 . . . f.sub.r, f.sub.n, then there must be a different set of filters for each. Changing the local oscillator frequency does not permit the receiver to be tuned to different sources, because if the local oscillator frequency is changed to bring the f.sub.2 sidestepped frequency to the center of the first filter, then the sidestepped 2f.sub.2, 3f.sub.2, etc. frequencies will not be centered in the filters appropriate to them. The second problem is that compensation for errors in the underground transmitter frequency (pulse rate) cannot be provided other than by widening filter bandwidths.